1. Field of the Invention
The invention relates in general to a filtration circulation system of a chemical mechanical polisher, and more particularly, to a circulation system which maintains a constant flow of slurry in a polisher to prevent the deposit of particles.
2. Description of the Related Art
Chemical mechanical polishing (CMP) is the only technique that provides a global planarization of devices for very large scaled integration (VLSI), or even ultra-large scaled integration (ULSI). In chemical mechanical polishing, a polishing apparatus similar to a grinder is adapted with an appropriate chemical agent to planarize uneven profiles of chips. A slurry is commonly used as a chemical agent in chemical mechanical polishing. The slurry comprises a colloidal silicon dioxide or a dispersoid of alumina mixed with the solution of potassium hydroxide (KOH) or ammonium hydroxide. The very highly abrasive particles in such a slurry and some added oxidant and organic solvent therein are used to facilitate the polishing process.
Therefore, the factors which determine the resultant planarization from chemical mechanical polishing include the components of slurry distribution and size of slurry particles, slurry supplying speed, and pH value of the slurry. In chemical mechanical polishing systems, a filter is installed for filtering slurry particles in a large size in order to reduce the scratch on the surface during polishing. However, while the polishing process stops or after a duration of operation, these large size particles deposit. In addition, while the polishing is in process, small size particles easily agglomerate into large size particles. The scratch on surfaces of chips during polishing is inevitable. Moreover, the deposit of large size particles blocks the filter and affects the slurry flow. Once the slurry flow is affected, scratches or damage to the chips become even more severe.
FIG. 1 shows a filtration system in a conventional chemical mechanical polisher. As shown in the figure, a slurry is controlled by a switch 10 to flow through a slurry pump 12. The slurry is then pumped by the slurry pump 12 to a slurry filter 14. After the particles of large size have been filtered by the filter 14, the flow of slurry is controlled by another switch 16 to flow to a polisher for performing a chemical mechanical polishing. While the polisher stops working, the switches 10 and 16 are shut, so that the slurry in the slurry filtration system 18 can not maintain a flowing and circulating state. The particles in the slurry therefore agglomerate into large particles. To avoid the deposit or agglomeration of particles, a deionized water (DI water) is used in the filtration system 18 to clean the residue slurry for polishing. Therefore, while performing chemical mechanical polishing, it is difficult to supply slurry into the filtration system 18 in a timely fashion. For example, the concentration of the abrasive particles is insufficient to performing polishing at the beginning of the process. A long duration is required to supply slurry with a sufficient concentration of small size particles. The stability and performance of chemical mechanical polishing processes are thus affected.